Compile-time checks definitely can't guarantee anything. You could still end up with null pointers from, for example, a library like gson using reflection to set fields. If you add "non-null" to the type descriptors inside class files (something like Kjava/lang/Object; instead of the usual Ljava/lang/Object;) and have the JVM enforce non-null-ness at runtime, you'll still have crashes caused by null pointers but earlier. Oh and you'll have to make an exception to allow these to be null during a constructor or static initializer invocation so it could actually initialize everything. It gets messy.

IIRC that proposition for value types makes them more akin to structs, without inheritance and dynamic methods. And I don't remember whether the fields are final, but given the current trend towards making everything as immutable as possible, they probably are.